I have not been able to find the answer to the following question. Hope someone more knowledgable than I can answer.

SAP calculations provide a design heat loss - what I need to establish is the ambient temperature used for this figure, i.e. at 'x' degrees centigrade outside, you need to add 'y' kw (design heat loss) to maintain the adjusted tempeatrue of 18.5 degrees inside.

Can anyone enlighten me as to the 'x' figure - my understanding was that this was -8 degrees?

Many thanks in anticipation

You can pick any winter design temperature that suits the location. -8 would be a little silly for me down in Kent, but not for someone up in Scotland I'd imagine. The total output of your heating system has to be your whole house heat loss coefficient x your temperature delta.

Whether you use -8, -5 or some other number isn't going to make a big difference to the number you get, unless you have a very energy hungry house.

I was looking at some heat pump literature and I read somewhere that the MCS say that the it must supply all the heat load 99% of the time. In other words you can exclude the coldest 1% or about 3 1/2 days each year. This, down here anyway, makes quite a difference, as does using modal temperatures, the real way to work out what is happening, rather than mean temperatures.
So all you need to do is do a Probability Density Function on your temperatures, convert to a percentage, exclude the bottom 1% and then see that where your starting point is.

So from the chart below (somewhere in Scotland), you can see that the lowest 1% is at -4°C, so you can do your calculation from -3°C.

Stones

If you think that -19 will be the 'norm' for your heating season, then size the system accordingly. Personally, where I come from the coldest one winter was -30 (RAF Shawbury I think?), but I would not recommend anyone using that datum for sizing their systems...

No one knows for sure what the winter temps will be in the next 25-30 years, so take a view and keep your fingers crossed that you were in the right ball park.
I tell our clients to use -3 external to 21 internal for sizing their systems.

Cheers